FIG. 28 is a perspective view of a multi-layer electronic element of the prior art. FIG. 29 is an enlarged sectional view of a junction between a side face of a stack constituting the multi-layer piezoelectric element and external electrodes. As shown in FIG. 28 and FIG. 29, the multi-layer piezoelectric element is constituted from a stack 110 and external electrodes 104 formed on a pair of opposing side faces.
The stack 110 is constituted from piezoelectric layers 101 and metal layers 102 stacked alternately one on another, but is formed in a so-called partial electrode structure where the metal layers 102 are not formed over the entire principal surfaces of the piezoelectric layers 101. The stack 110 has a no-electrode section where the internal electrodes 102 are not formed on the principal surface of the piezoelectric layer 101, and the no-electrode sections are disposed so as to alternately adjoin the pair of external electrodes 104. In this constitution, the internal electrodes 102 are exposed in every other layer on the opposite side faces of the stack 110 and are connected in every other layer to the pair of external electrodes 104, that are formed on the opposing side faces of the stack 110. Inactive layers 109 are formed on either side of the stack 110 in the stacking direction.
The external electrodes 104 are generally formed by applying an electrically conductive paste, which includes an electrically conductive material such as silver, onto the side faces of the stack 103, and baking the assembly (refer to, for example, Japanese Unexamined Patent Publication (Kokai) No. 2000-332312, Japanese Unexamined Patent Publication (Kokai) No. 2000-31558, Japanese Unexamined Patent Publication (Kokai) No. 2005-174974).
In case the multi-layer piezoelectric element is used as a multi-layer piezoelectric actuator, lead wires 106 are fastened onto the external electrodes 104 by soldering, and a predetermined voltage is applied across the pair of external electrodes 104 via the lead wires 106 so as to drive the multi-layer piezoelectric element. Particularly, in recent years, it has been practiced to apply an electric field of higher intensity and operate the device continuously over a long period of time, since a compact multi-layer piezoelectric element is required to maintain a large amount of displacement under a high pressure.    Patent document 1: Japanese Unexamined Patent Publication (Kokai) No. 2000-332312    Patent document 2: Japanese Unexamined Patent Publication (Kokai) No. 2000-31558    Patent document 3: Japanese Unexamined Patent Publication (Kokai) No. 2005-174974